Underwater diving is a growing activity around the globe, whether it is for commercial, recreational and military purposes. Diving is also a hazardous activity, and serious or fatal accidents occur regularly. One Japanese study conducted in 2000 determined that every hour of recreational diving is 36 to 62 times riskier than driving an automobile.
There are a myriad of dangers that a diver faces during a dive. These dangers/hazards can be categorized as follows—specific diving environment (silt, overhead environment (cave, wreck, etc.) low visibility, darkness, etc.) use of breathing equipment (carbon monoxide contamination of breathing gas, oxygen partial pressure in breathing gas is too low, loss of breathing gas supply, excessive carbon dioxide in breathing gas, etc.), pressure changes during descent, pressure changes during ascent (lung overpressure, middle ear overpressure, etc.), breathing gases at high ambient pressure, pre-existing physiological and psychological conditions of the diver, failure of diving equipment other than breathing apparatus, etc.
Most of the hazards listed are dormant or potential, with only a theoretical risk of harm. Once the hazard becomes active, it creates an emergency situation.
One of the dangers facing divers is knowing their precise position underwater. Most scuba divers carry a compass. A compass alone does not help but rather requires the diver to use the compass bearings with known landmarks and the diver to estimate their distance from the landmark. In situations of strong currents or low visibility it will be difficult for the diver to locate known landmarks, let alone estimate the distance to the landmark.
Tethering the diver allows topside personnel to monitor and instruct a diver on their location. However, the diver is encumbered by the presence of an attached tether restricting the spatial length of the dive to the length of the tether. Other methods of positioning is the use of lines that can either be laid out between landmarks or attached to a weight in order to swim circular search patterns. Although these methods work, they also require the diver to be tethered and only provide a position relative to local objects.
Another common danger that faces divers, as mentioned above, is the pressure changes during ascent. Dive computers, generally worn on a diver's wrist or attached to the diver's suit by a retractable cord, address the same problem as decompression tables, but are able to perform a continuous calculation of the partial pressure of inert gases in the body based on the actual depth and time profile of the diver. Many dive computers also provide additional information to the diver including air and water temperature, data used to help prevent oxygen toxicity, a computer-readable dive log, and the pressure of the remaining breathing gas in the diving cylinder. As the dive computer automatically measures depth and time, it is able to warn of excessive ascent rates and missed decompression stops. However, because the dive computer is worn on the wrist or attached to the diver's suit, the diver may not be aware of any warnings displayed on the dive computer or may not hear an audible alarm.
Companion divers may not be aware of impending danger to a fellow diver without some visible warning, especially if the danger relates to irregularities in the diver's breathing loop. Also, there may be circumstances where a diver is aware of an impending danger to another diver and needs to gain the attention of the diver. Traditional hand signals may be missed or misunderstood, and audio warnings may not be heard because of the surrounding environment and head gear worn by divers.